In those data transmission systems which rely upon a DSB-QC modulation technique, the sequence of bits to be transmitted is first divided into groups of Q bits and each of these groups is made to correspond to one of 2.sup.Q complex numbers, or complex symbols. The set of 2.sup.Q symbols is usually represented on a complex plane by mapping thereon the points having these symbols as coordinates. In general, these points are collectively referred to as the signal constellation. The symbols are then transmitted one at a time at instants which have a regular T-second spacing and are called signaling instants. Each symbol is transmitted by causing an amplitude value of each of two quadrature carriers to correspond, respectively, to its real and imaginary parts. These two carriers are then combined and applied to the input of the transmission channel.
The function of the transmission channel is to provide at its output, which is connected to a data receiver, a signal relatively similar to the input signal applied thereto. Mainly due to cost considerations, telephone lines are frequently used as transmission channels. However, such lines, while satisfactory for voice transmission purposes, become inadequate when used to transmit digital data at speeds equal to or higher than 4800 bits per second (bps) with a very low probabilitity of error. Telephone lines cause impairments which affect the quality of the signals being transmitted and make it difficult to correctly detect the transmitted data. These impairments mainly include amplitude and phase distortions that create an interaction between successive signals. This interaction is known as intersymbol interference. In high-speed data transmission systems, the receiver is generally provided with an automatic adaptive equalizer to minimize the effects of intersymbol interference prior to the data being detected.
The type of adaptive equalizer that is the mpst widely used in those data transmission systems which rely upon a DSB-QC modulation technique is the complex transversal equalizer an exemplary embodiment of which is described in French Pat. No. 73-26404 (publication No. 2,237,379). In such an equalizer, each of the in-phase and quadrature components of the received signal is applied to the input of a pair of transversal filters whose outputs are then combined to generate the in-phase and quadrature components of the equalized signal. The tap gains of these filters, which constitute the tap gains of the equalizer, are automatically adjusted to meet a given performance criterion. Before transmitting any data, it is necessary that the values of the tap gains be as close as possible to optimum values. To this end, provision is made for a training period during which, before sending any data, a known training sequence is transmitted and is then analyzed by the receiver which derives therefrom an initial adjustment of the tap gains of the equalizer to values that are as close as possible to the optimum values. The tap gains are then continually adjusted during transmission of the data.
In those data transmission systems which are configured as a multipoint network, several data terminals exchange data over a common transmission line. Each terminal is connected to the common line through a modem that includes a data transmitter and a data receiver. These terminals have varying degrees of intelligence, and in general one of them, which is often a computer, controls the entire network, with the transmission of data taking place between this computer and the other data terminals. Usually, the modem associated with the computer is referred to as the master modem while the other modems are called the slave modems. In general, a network training procedure is carried out once a day before the first transmission of data takes place. During this procedure, the master modem transmits synchronizing signals over the common transmission line to condition the receivers of the slave modems to receive the data to be subsequently transmitted. These synchroning signals include a known training sequence that serves to train the equalizers of the slave modems. However, it may prove necessary in some cases to train the equalizer of a slave modem after completion of the network training procedures. For example, a slave modem may happen to be in a power off condition at the time the network training procedure is carried out and as a result fail to receive the training sequence, or it may be conditioned through said procedure, but subsequently lose equalization because of some degradation of the transmission line characteristics.
Several methods of training the equalizer of a receiver in such cases have been proposed in the prior art.
In one of these methods, the master modem periodically transmits a training sequence to permit training the equalizers of all slave modems. In another known method, any slave modem which has lost equalization alerts the master modem which responds by transmitting a training sequence intended for the equalizer of that particular slave modem.
In both of these prior art methods, the receiver of the slave modem knows the transmitted training sequence and can fairly easily derive an initial adjustment of the equalizer from the received training sequence. However, in both methods, the operation of the entire network is adversely affected since no data transmission can take place while the training sequence is being transmitted.
In a third method proposed in the prior art, means are provided in each slave modem for storing several sets of equalizer tap gain values which are successively tested whenever a loss of equalization occurs. Obviously, a limited number of such sets can be used, and it may not be possible to retrieve the equalization in the event of significant distortions being introduced by the transmission line.
French patent application No. 78-18478 filed by the present applicant June 13, 1978, discloses a method of training the equalizer of a receiver in a transmission system that relies upon a quadrature amplitude modulation technique; this is done by means of an unknown data signal, which makes it possible, in particular, to train the equalizer in the cases described above. In accordance with the method described in said patent application, in a data receiver wherein, under normal operating conditions, the data are detected with respect to a first signal constellation, which is the one used by the transmitter in said system, training of the equalizer is achieved by detecting the data with respect to a second signal constellation that comprises fewer points than the first one and by adjusting the tap gains of the equalizer by means of an error signal produced as a result of the detection operation performed with respect to the second signal constellation. This solution yields very good results, but requires that the signal constellation used by the transmitter be known to the receiver. In any data transmission system, the data rate can vary from time to time as a result, for example, of impairments caused by the transmission channel. In systems using DSB-QC modulation, the data rate is generally varied by varying the signal constellation being used rather than the signaling rate. In that case, a slave modem which has lost equalization and must use any signals present on the common transmission line for training purposes does not known exactly the signal constellation being used by the master modem.